1. Field of the Invention
This invention relates to heat transfer devices and more particularly to heat pipe systems.
2. Description of Related Art
One of the limitations of a heat pipe is its length. That limitation is governed by the amount of heat it can transport per unit time. Another limitation of a heat pipe is its ability to "transform", i.e. to redistribute the heat flux from one end of a heat pipe to the other end of the heat pipe. By removing the length limitation and by making a flux transformer possible, the opportunities for heat pipes are enhanced by orders of magnitude in applications such as space technology where thermal control of power management systems pose significant challenges.
Some recent work pertaining to heat pipes includes a liquid metal heat pipe which is designed to be lightweight and capable of production in large quantities at relatively low cost, as described by Vincent C. Truscello, in "Liquid Metal Heat Pipe," DOCN 000009921, Jet Propulsion Laboratory, Pasadena, Calif. (1993). The key to light weight is an inner structure made of 2 mil foil with a pattern of 3.5 mil diameter holes. Another way of achieving light weight is to consider different diameter tubes since length is no longer a limitation. In another application, current heat rejection systems for spacecraft are made primarily of heat pipes and pumped loops. In "Design and Analysis Code for Combined Pumped-Loop, Heat Pipe Radiator," DOCN 0000019626, Lewis Research Center, Cleveland, Ohio (1989) it is stated that although a heat pipe system often has a lower mass than a comparable pumped-loop system, it is often impossible to configure systems to use heat pipes alone. One of the reasons for this is the length limitation. Here again, coupled heat pipes may alleviate that limitation.
It has been shown there that the high capacity heat pipe radiators offer several advantages over conventional heat-rejection methods as described by Steven D. Glenn in "Space Station Heat Pipe Advanced Radiator Element II," Research and Technology Report, Lyndon B. Johnson Space Center, Houston, Tex. (1991)
It has been demonstrated that the innovative use of heat pipes in different configurations and for different purposes allows them to provide the same performances as a conventional air conditioning system with three times the capacity and size in "Mobile Heat Pipe Air Conditioner", DOCN 000018860, John F. Kennedy Space Center, Fla. (1993.) In addition, they state its capability of maintaining the supply air within required specifications. That heat pipe air conditioning system permits savings within the range from 65% to 70% over conventional air conditioning systems.
An article by S. Oktay "Beyond Thermal Limits in Computer Systems Cooling" Cooling of Electronic Systems, Editors S. Kakac and H. Yuncu, The Netherlands (1994) includes a description of the related art as well as a description of aspects of this invention. That description is incorporated herein by reference. Some of the material described therein is included herein as well.
In electronics, the surface area of devices has been shrinking so fast that cooling has become more and more of a challenge. The force behind shrinking dimensions is the drive to reduce electrical signal propagation times for higher and higher system performance. It is that linkage between thermal and electrical parameters which sheds light on the nature of thermal limits, if any.
Currently the computer industry as well as the computer itself are being reshaped. From the mainframes of the previous decades to the client-servers of today and on to the projected PC in every home in the year 2001, the trend is for information not only at the finger tips, but also, one may say, surrounding the body. Consider, for example, a computer in the form of a "shawl" slung over the shoulder where an integrated computer-fax-telephone machine is comfortably cooled not exactly by perspiration, but, possibly, by transpiration.
The "downward mobility" from general purpose computers to desktop computers to mobile computers is driven by the ever increasing improvements in the semiconductor technology, namely the ability to be able to squeeze many more transistors into a given area.
Accompanying the dizzying improvements in the hardware technology there have also been some significant improvements, albeit at a slower rate, in the software arena, such as parallel programming. Thus a multiplicity of single chip processors supported by appropriate parallel processing programs encourages "upward mobility" to economically attractive "Next Frames", or Next Generation Mainframes, and even to application specific super computers, for that matter. Hence, if improvements in hardware technology alone accelerate "downward mobility" towards smaller computers and mobile computers, coupling that with improvements in parallel processing encourages possibly affordable "upward mobility" as well.
In terms of cooling, all this has significant implications. How does one cool a "10 MIP computer shawl" slung over a shoulder while carrying inside it chip(s) with hundreds of thousands of circuits? How is a 500 MIPS DeskTop of the mid-90's to be cooled without the noise of cooling blowers polluting the quiet of the office? How is a 1000 MIPS or, say, a 10 BIPS NextFrame of year 2001 to be cooled?
Whether it is "downward," or "upward" mobility in computers, what is clear is that miniaturization of technology is driving to higher power densities at the junction level, while the reduction in absolute power per function is reducing the total heat dissipated by the system. In other words, the trend is towards far more MIPS/MW, or for more work and less "hot air." A decade ago, computers could perform about 100 MIPS per MegaWatt of energy and in the year 2000 they are expected to do more with less about 1000 MIPS, or 1 BIPS per MegaWatt. With CMOS technology, this could change dramatically to tens of BIPS per MegaWatt.
This trend has interesting implications. While it is becoming "easier" to dissipate heat at the system level, because there is less of it to eliminate per function, the challenge of cooling is being pushed more and more towards the source where the heat is generated, i.e., the junction and the chip. Taking some liberties with the symbolic "computer shawl" of the year 2001, it would not be too desirable to mount a huge heat sink on a single-chip-processor computer with a blower blowing at 100 liters per second while carrying a shawl around the shoulder of the user. Nor would it be attractive to mount a relatively small micro-channel water-cooled heat sink on the chip and then again strap a jug of water on the back to cool the shawl. In today's mainframes or other relatively large systems of several chips, the usual mechanical aids for cooling the chips could be buried inside the "big-footed" systems. However, just like the trend for smaller devices on chips, and fewer chips in a system and hence physically much smaller but much more powerful systems, the same trend for cooling is needed where the cooling hardware is commensurate with the size of the overall system. To this end, it is helpful to examine the thermal limits in electronic packaging.
Heat pipes are well known devices used for cooling. FIG. 1 shows a perspective view of a prior art heat pipe device comprising a sealed container 10 in the form of a cylindrical tube 12 cut away for convenience of illustration. (Heat pipes typically consist of a sealed container (usually a cylindrical tube). The tube 12 is lined internally, on its inner walls, with a cylindrically shaped wicking material 14. The container is evacuated and backfilled with just enough liquid coolant to fully saturate the wicking material 14.
When the first (proximal) end of the container 10 known as the evaporator 16 is heated, the working fluid coolant at the evaporator end of the container 10 is vaporized into a coolant gas, thereby absorbing substantial heat because of the change of state from liquid to gas. The resulting high vapor pressure resulting from the vaporization drives the coolant gas (vapor) in the vapor flow direction 22 from the evaporator section 16 through the adiabatic section (arrow 22) to the condenser section 18 at the distal end (known as the condenser 18) of the container 10.
At the condenser 18, when the condenser 18 is at a lower temperature than the coolant gas, the coolant gas (vapor) condenses back into the liquid state thereby releasing the latent heat of vaporization from the coolant gas to the condenser 18. The capillary forces in the wicking material 14 then pump the coolant liquid back along a path in the direction indicated by liquid flow arrow 24 to the evaporator end 16 of the container 10.
The cycle is continuous and is repeated so long as heat is supplied to the evaporator and the condenser condenses the vapor. Since this embodiment uses the latent heat of vaporization of the working fluid (coolant) as opposed to sensible heat, the temperature difference between the heat source at the opposite ends represented by the evaporator 16 and the condenser 18 is very small.
Consequently, the equivalent thermal conductivity of such a heat pipe can be several orders of magnitude greater than that of a device such as a solid copper rod of the same dimensions. The heat transport capability of the heat pipe is a function of the ease of circulation of the working fluid (coolant). There is a definite relationship between the length of a heat pipe and the load it can carry before the capillary pumping mechanism fails. Heat pipes use coolants such as ammonia, fluorocarbon (Freon-21 trandemark of E.I. dupont de Nemours), methanol, water and other fluids with suitable boiling points among other characteristics such as high surface tension.
As to wicking materials, large internal pores are necessary in a direction normal to the liquid flow path to minimize liquid flow resistance. Small surface pores develop high capillary pressure and a highly conductive heat flow path. Homogeneous wicks and composite wicks are available. The types of wick materials available include wrapped screens, sintered metal, axial grooves, annular and crescent wicks, and artery wicks. Composite wicks include composite, screen-covered groove, slab and tunnel wicks.